Lifetime exposure to sex steroids is a major contributor to breast cancer risk. The risk-benefit ratio of prevention strategies that target the endocrine axis would be significantly improved if it were possible to assess risk for hormone sensitive breast cancer. Nipple fluid hormone levels are attractive candidate markers of breast cancer risk. Our pilot studies provide preliminary data that nipple aspirate fluid (NAF) levels of sex steroids (estradiol and progesterone) are potential biomarkers of breast cancer risk in both pre and postmenopausal women. We show that these markers are significantly higher in NAF than in serum, are stable over time, and are biologically relevant, as reflected by the strong correlations of NAF estradiol levels with the content of estrogen response proteins in NAF. Additionally, we show that the hormone content of NAF reflects the systemic environment, since hormone levels rise with the use of hormone replacement therapy, are low in oral contraceptive users, and rise after cessation of oral contraceptives. Finally, preliminary analyses show, for the first time, a correlation of NAF estradiol and progesterone with estimated Gail risk. The primary hypothesis to be examined in this proposal is that NAF estradiol levels are markers of risk for hormone sensitive breast cancer. To accomplish this, we are proposing a case-control study of 600 women: 300 newly diagnosed breast cancer cases aged 40 to 60 from the Lynn Sage Breast Center, and 300 controls from the adjacent Mammography Screening Center of Northwestern. Cases and controls will be matched for age and menopausal status. We will obtain NAF, blood samples, and mammographic density measurements. NAF assays will include sex steroids (estradiol and progesterone), their precursors (estrone, estrone sulphate, androstenedione dehydroepiandrosterone and testosterone) and related proteins (epidermal growth factor, cathepsin D, pS2, and prostate specific antigen). We will perform sub-analyses to examine the effect of specific breast cancer risk factors, menopausal status, and race on the association of NAF hormones and breast cancer risk. We will seek corroborating evidence for the biological importance of NAF hormone levels by examining the NAF content of estrogen and progesterone related proteins. By examining the hormone receptor (HR) status of the cancers among cases, we will test the hypothesis that NAF hormone levels are more strongly related to HR positive breast cancers. Finally, we will assess the degree to which NAF hormones explain mammographic density in cases and controls. This project, evaluating breast intracrinology, mammographic density, and cancer risk will provide data on the validity of NAF hormones as new breast cancer risk markers, which can potentially be modulated by preventive interventions and therefore may serve as surrogate endpoints to assess their effects.